The present invention relates to a semiconductor device of so-called chip-on-chip (COC) type for connecting plural semiconductor chips face to face electrically. More particularly, the present invention relates to a structure of a semiconductor device capable of bonding two mutually confronting semiconductor chips without applying a high temperature or high pressure between the semiconductor chips when connecting together by way of a bump, and preventing creeping of resin up to the bump and lowering of adhesion strength even when forming a protective film of polyimide or other resin on the surface of semiconductor chips for the purpose of cutting off alpha-rays and so on.
Hitherto, when a semiconductor device is composed by combination of circuits, such as combination of a memory element and its logic circuit, in order to reduce the area occupied by a solid structure, decrease the parasitic capacity of high frequency circuit, or share part of the circuit (for example, a part of the memory element is shared and the drive circuit portion is changed depending on application), and in a large scale integration in which it is difficult to form in one-chip structure because the severity of manufacturing condition differs in circuit portions, a semiconductor device of COC type is used, in which other semiconductor chip (child chip) is connected on one semiconductor chip (parent chip) by manufacturing a semiconductor circuit composed of a plurality of chips.
In such configuration of semiconductor device, for example, as shown in the bonding process of two semiconductor chips 1 and 2 explained in FIG. 5, one semiconductor chip 1 is fixed on a heated substrate stage 51, other semiconductor chip 2 is fixed on a mount head 52, the mount head 52 is pressed to compress bump electrodes 11 and 21 made of Au or the like of the two chips, and these bump electrodes 11 and 21 are heated to about 450xc2x0 C. and connected. The material of the bump electrodes 11 and 21 is generally a metal material of a higher melting point such as Au than solder because the semiconductor device is mounted on the substrate by soldering.
In the semiconductor device of such COC type, Au or other high melting point metal material is used as the bump in order to connect plural semiconductor chips beforehand, and therefore favorable electric connection is not obtained unless pressed by heating to about 450xc2x0 C. When heated to such high temperature when connecting the bump, the semiconductor substrate itself becomes higher than 450xc2x0 C., and the circuit elements (transistors and other elements composing the semiconductor device) formed on the semiconductor substrate are also heated to a high temperature, and the element characteristics may vary.
Thus, in the semiconductor device of COC type, when connecting the parent chip and child chip through bump, a high temperature of about 450xc2x0 C. and a pressure must be applied, and element characteristics may vary, and in order to avoid such variation, the present inventor devised, for example, a semiconductor device in a structure of adhering by an Auxe2x80x94Sn alloy layer by forming an Sn coat film on an Au bump, and disclosed it in Japanese Patent Application No. 2001-21113.
On the other hand, for the purpose of cutting off alpha-rays, a protective film may be formed on the semiconductor chip surface by using resin such as polyimide, and when applying the polyimide, as shown in a magnified view of the junction in FIG. 4, a resin 18 may creep up to the bump 11 by surface tension to stick to the surrounding surface of the bump 11, or a thin resin film may be formed on the entire surface of the junction. When such resin 18 sticks to the surface of the bump 11, the contact area of the bump 11 is decreased, or a fillet 3a of an Auxe2x80x94Sn alloy layer 3 may not be formed sufficiently as shown in FIG. 4, and the adhesion strength is weak and the reliability of the junction is lowered. In particular, there is a problem that the reliability of adhesion is lowered significantly, when the resin may creep up to the bump surface by surface tension, and a thin resin film is formed, because hardly any pressure is applied between them when bonding, and adhesion is made by alloying the bump with the low melting point metal.
The present invention is devised in the light of the above background, and, relating to a semiconductor device of COC type, it is hence an object thereof to obtain a semiconductor device being free from effects of the mounting temperature of the semiconductor device, and having a structure capable of maintaining a high bonding strength at an electrode junction even when a protective film of resin such as polyimide is formed on a chip surface, while connecting the electrode terminals of semiconductor chips without deteriorating the characteristics of semiconductor chips by the temperature applied at the time of bonding.
The semiconductor device of the present invention is a semiconductor device includes; a first semiconductor chip, a second semiconductor chip, and a bump provided on at least one of the first semiconductor chip and second semiconductor chip, the bump being disposed at a junction portion of the first semiconductor chip and second semiconductor chip, wherein the bump is made of a first metal, a top surface of the bump is bonded through a second metal having a lower melting point than that of the first metal, and a step lower than a central portion is formed at least in part of an outer periphery on the top surface of the bump.
Herein, the semiconductor chip is not limited to an integrated circuit (IC), but includes discrete components such as transistor, diode and capacitor. The second metal is not a single metal alone, but includes a eutectic alloy, which is meant to include an alloy formed between the first metal and a third metal in the bonding process by disposing the third metal on the first metal surface.
In such configuration, even in the case of forming a protective film of polyimide or other resin on the surface of a semiconductor chip for the purpose of cutting off alpha-rays, when the resin is applied by spin coating or the like and creeps up to the bump by surface tension, it is stopped at the step portion of the bump, and the resin does not creep up to the top surface, that is, the adhesion surface of the bump. As a result, it avoids conventional problems, such as narrowing of adhesion surface of the bump, lowering of adhesion strength due to a thin resin film formed on the bump surface, or increase of contact resistance.
More specifically, the first semiconductor chip has a first bump, the second semiconductor chip has a second bump of which peripheral length is shorter than that of the first bump, the step is formed around the first bump, and the first bump and second bump are bonded through the second metal layer so that a fillet may be formed, and therefore even if a protective film of polyimide or the like is formed on the first semiconductor chip surface for the purpose of cutting off alpha-rays, the resin film does not creep up to the fillet forming surface, and a fillet can be formed in a wide area, so that a very stable bonding is obtained.
When the first metal is Au and the second metal is an Auxe2x80x94Sn alloy, it is possible to be adhered at a relatively low temperature without applying pressure, and the melting temperature may be higher than the soldering temperature to the mounting substrate, and therefore the junction portion is not detached at the time of mounting, and a very stable bonding is obtained. Moreover, when a protective film formed by curing a liquid resin is disposed at least around the bump of the semiconductor chip having the bump in which the step is formed, the resin is prevented from creeping up to the junction surface of the bump, and the effect is outstanding in particular.